1. Technical Field
This invention relates in general to micro-electro-mechanical systems devices and in particular to applying a micro-electro-mechanical wobble motor as a radio frequency transceiver component.
2. Background Art
Radio designers would like to create a highly integrated radio which can switch between many different radio bands. The radio would be packed as a single-chip or a two-chip stack, if possible. Such a transceiver would allow a single, compact cellular handset to be used on many different worldwide cellular systems.
One approach to creating this is to use micro-electro-mechanical systems (MEMS) techniques to create components on cooperatively positionable chips that allow the radio to be rapidly and electronically reconfigured for various transmit and receive frequencies. These components would include switches, variable capacitors, inductors and other passive or active elements. Typical steps for reconfiguration might include switching of various fixed-value passive devices (i.e. capacitors, inductors, resistors, etc.) into and out of the radio circuitry, and/or adjusting the value of one or more of these devices over some finite but preferably wide range.
In principle, MEMS could create the necessary switches and variable capacitors. It can also be used to integrate many or all of the passive components onto a single substrate, which may or may not be the same substrate used by the rest of the radio circuitry. However, current MEMS switches and variable capacitors have suffered from various problems. In particular, these switches have required higher than desirable voltage levels for actuation. This is especially true for capacitively-actuated, cantilever beam-type switches where the gap is initially rather large, thus requiring higher voltages. In addition, the only force acting to release this type of switch is the spring force of the cantilever, which can lead to stiction problems since the beam must be soft enough to be easily pulled down.
As for variable capacitors, one prior art approach made the capacitor out of a thin membrane or beam suspended over a base surface. The separation distance between the beam and the base is controlled electrostatically. Unfortunately, these variable capacitors can have problems related to the non-linearity of gap versus applied voltage. They are also susceptible to undesirable changes in capacitance as a result of acceleration and microphonic effects.
Wobble motors are known in the MEMS field and come in at least two forms. The first type operates entirely within a single plane (not shown). It uses a rotor with a slight amount of clearance either on the inner (shaft) diameter, or around the outside diameter of the rotor. In this design, the rotor can be pulled in a finite number of radial directions through the use of a series of electrodes or electromagnet elements positioned around the rotor. When one of these electrostatic or electromagnetic "poles" is energized, the rotor moves radially toward that pole until the small amount of clearance has been eliminated. Subsequently, the poles around the rotor can be energized in series around the circle. This causes the rotor to roll around the center shaft or the external contact point between the outside of the rotor and the stator, depending on the type of wobble motor. This rolling motion causes a very slight rotation of the entire rotor each time the energization of the poles completes one "electrical" cycle.
The second type of wobble motor uses a flat circular rotor which sits atop a small bump at its center. If the rotor were to remain flat, there would be a small gap between the bottom edge of the perimeter of the rotor and the substrate below. With this type, each of the poles which are situated around the rotor can be energized to pull the edge of the rotor nearest that pole down until it is in contact with the substrate. The pole forces act in directions which are parallel to the rotational axis of the wobble motor, rather than in radial directions as is done with the previous type. Energizing the poles in series will cause the contact point between the rotor and the substrate to rotate around the circle. Because of the slight tilting angle of the rotor, the radius of the circle of contact created by the successive contact points between the rotor and the substrate is slightly smaller than the radius of the rotor itself. Therefore, as long as there is no slippage at the contact point, the rotor will rotate very slightly each time the energization of the poles complete one cycle.
A critical aspect of both types of wobble motors is that they can function with extremely small gaps between the driving poles and the rotor. For example, with the second type, if the rotor is tilted toward one edge at "3-o'clock", it would not be practical to directly switch the rotor so that it tilted to the 9-o'clock position since the gap at the 9-o'clock electrode is too large and would require a large voltage. However, the gaps at the 2-o'clock and 4-o'clock electrodes can be very small. These gaps would be just slightly larger than the minimum allowable gap which occurs at the 3-o'clock position as it is being driven. Therefore, the way to move the rotor so that it tilts toward 9-o'clock is to sequentially step through the drive electrodes in either direction from 3-o'clock to 9-o'clock. The key point is that the gap at each electrode is very small when that electrode is to be energized. Thus, high voltages are not required. Furthermore, stiction should be much less of a problem because the motor does not rely on a spring which stores energy to be able to break a contact. Instead, the rotor is actively and strongly driven from position to position.
The frequency of operation (of these devices) is generally achieved by the selection of electrical values of related components including one or more resistors, inductors and/or capacitors. To change frequency selection requires the change of at least one of the components previously employed for a first frequency. The change can be achieved by switching out or removing the desired component(s) and switching in or inserting the desired replacement components. Alternatively, the change can also be achieved by varying the value of one or more components.